Conventionally, there is an electric compressor incorporating a DC motor, as an electric compressor included in a refrigeration cycle of a refrigerator. This electric compressor operates to circulate a refrigerant according to an internal temperature to keep food stored in the refrigerator at an appropriate temperature. Also, in recent years, there is known a technique in which a DC motor of an electric compressor is PWM-controlled, thereby achieving energy saving (e.g., see Patent Literature 1).
Patent Literature 1 discloses a running control device of the refrigerator, including a set temperature detecting means which detects a set temperature, an internal temperature detecting means which detects an internal temperature of the refrigerator, and an outside air temperature detecting means which detects a temperature of an outside region of the refrigerator. This control device sets an operational rotating speed of the electric compressor in multiple stages according to a difference between the internal temperature and the set temperature. Patent Literature 1 discloses that the control device sets the rotating speed as follows: when the temperature difference is equal to or greater than 5 degrees C., the rotating speed is 5400 rpm, when the temperature difference is in a range of 5 to 2 degrees C., the rotating speed is 3600 rpm, when the temperature difference is in a range of −2 to 2 degrees C., the rotating speed is 1800 rpm, and when the temperature difference is equal to or less than −2 degrees C., the rotating speed is 0. Patent Literature 1 also discloses that the control device changes a minimum rotating speed of the electric compressor based on the outside air temperature detected by the outside air temperature detecting means.
Thus, the control device disclosed in Patent Literature 1 is intended to optimize the set rotating speed of the electric compressor, by obtaining the internal temperature and the outside air temperature. That is, a magnitude of the difference between the internal temperature and the set temperature, and whether the outside air temperature is high or low, correlate with a magnitude of a load (cooling load) of the electric compressor. Therefore, if a detailed change status of the internal temperature and the outside air temperature are obtained, it becomes possible to decide appropriate rotating speeds which can make the internal temperature closer to the set temperature while considering a magnitude of the cooling load.